25th International Meeting on Organic Geochemistry IMOG 2011

P-498
Molecular and isotope characterization of soil lipids along a
savannah (C4)/eucalyptus (C3) chronosequence (Pointe-Noire,
Congo)
Thanh Thuy Nguyen Tu 1 , Mercedes Mendez-Millan 3 , Céline Egasse 1 , Sylvie Derenne 2 ,
Bernd Zeller 4 , Delphine Derrien 4 , Jeremy Jacob 5 , Christine Hatté 3
1 CR2P Université Pierre & Marie Curie, Paris, France, 2 BioEMCo Université Pierre & Marie Curie, Paris,
France, 3 LSCE CNRS, Gif sur Yvette, France, 4 BEF INRA, Nancy, France, 5 UMR6113 CNRS, Orléans,
France (corresponding author:ttnguyen@snv.jussieu.fr)
The surfaces dedicated to forestry
plantations are rapidly increasing throughout the
world, especially in the tropics [1]. The impacts of
such a rapid afforestation on global carbon cycle and
on soil that constitutes one of its key compartment, is
therefore a critical issue [2]. Stable carbon isotope
characterization of C4/C3 chronosequences has
proven useful in assessing the dynamics of organic
carbon in soils [3]. Lipids are important contributors to
organic matter in soils since they can influence
aggregate stability, water retention and fertility.
Nevertheless, little is known on the effects of
afforestation on lipid composition and dynamics in
tropical soils. The eucalyptus plantation located in the
area of Pointe-Noire (Republic of Congo) is a prime
field to study the impact of afforestation on organic
carbon dynamics. Indeed, a eucalyptus forest (C3)
has been progressively planted on savannah (C4)
during the last 30 years [4]. Lipids were solventextracted
from the top soil (0-20 cm) of stands planted
for 7, 17, 30 years, and of a reference plot of the initial
savannah. They were also extracted from the main
savannah species and from eucalyptus.
Lipids accounted for 0.3 to 0.9 wt % of the
dried soils. Gas chromatography-mass spectrometry
analyses of soil lipids revealed a complex mixture
comprising more than 100 identified constituents,
belonging to diverse chemical families (e.g. fatty
lipids, triterpenes, sterols, glycerols). Most of these
molecules correspond to plant components,
emphasizing the importance of this source of organic
matter to soil lipids. Afforestation did not appear to
affect significantly bulk lipid yields. Most of the
identified components are present all along the
chronosequence. Nevertheless, quantification of the
main compounds revealed, for some of them,
significant trends related to the vegetation changes:
(i) decrease of savannah grasses markers (very long
chain fatty lipids, pentacyclic triterpene methyl
ethers), (ii) increase of eucalyptus contribution (long
chain fatty lipids) and (iii) increase of microbial marker
(cholesterol). Compound-specific � 13 C analyses were
performed (1) on odd carbon chain length n-alkanes
from 25 to 31 carbon atoms that are specific to higher
plants and common to both vegetations and (2) on
savannah grass markers. After 30 yrs of eucalyptus
crop, the � 13 C of soil C25 and C27 n-alkanes displayed
typical trend of a C4/C3 vegetation shift with a
depletion of 4.4 ‰ and 6.4 ‰. Surprisingly, the C29 nalkane
� 13 C is of -30.5 ‰ for all stands, pointing to
additional sources for this compound. The isotopic
signature of the C31 n-alkane showed no incorporation
from the eucalyptus plant, as for pentacyclic triterpene
methyl ethers. Compound-specific 14 C analyses are
under progress to precise the origin and the residence
time of the different markers in soil.
This work is part of the DynaMOS ANR project.
References
[1] FAO, State of the World's Forests, Food and
Agriculture Organization of the United Nations, Rome
(2003) 151 pp.
[2] Garcia-Quijano J.F., Deckmyn G., Moons E.,
Proost S., Ceulemans R. and Muys B. (2005). An
integrated decision support framework for the
prediction and evaluation of efficiency, environmental
impact and total social cost of domestic and
international forestry projects for greenhouse gas
mitigation: description and case studies. Forest Ecol.
Manage. 207 pp. 245–262.
[3] Cerri C., Feller Ch., Balesdent J., Victoria R. and
Plenecassagne A. (1985) Application du traçage
isotopique naturel en 13 C à l'étude de la dynamique
de la matière organique dans les sols. C.R. Acad.
Sci., Sér. D, 300 Sér. II 9, pp. 423–427.
[4] Bernhard-Reversat F. (1993) Dynamics of litter
and organic matte rat the soil-litter interface in fastgrowing
plantations on sandy ferralitic soils (Congo).
Acta Oecol. 14, pp.179-195.
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